Yep, they could. AFAIK there’s already a law in Spain that allows municipal governments to increase property tax on owner that have more than 5 apartments by up to 100% or something. I have no idea if it’s actually used.

The problem is that many villages and small towns are depopulating and you have plenty of empty properties there that simply no one wants to buy. There are no jobs in those towns so people are migrating to bigger cities and the apartments in those cities are not built fast enough to absorb the increase.

So the central government lets local governments to decide if they want to implement those policies or not (because they should know best if it’s needed) and a lot of local governments in bigger cities are right wing so they choose not to. The other issue is that the current governments needs Catalans to pass any laws and Catalan parties traditionally protect the rights of property owners. It was clear from day 1 that this coalition is too fragmented to govern effectively. They basically spent the last couple of years fighting for survival instead of governing.